Fidel Castro appears with Venezuela's Hugo Chavez, left, and brother Raúl Castro in a photo released in June.

HAVANA, Cuba (CNN) - President Obama misinterpreted Cuban President Raúl Castro's offer to start talks with the United States, Castro's brother Fidel said Wednesday, appearing to dismiss the U.S. leader's call for Cuba to release political prisoners.

In an essay published in state-run newspapers Wednesday, the ailing revolutionary leader said the people Washington calls political prisoners are "in the service of a foreign power that threatens and blockades our homeland."

Fidel Castro's comments come after signs of a thaw in the decades-old impasse between the United States and the communist-ruled island to its south.

Obama lifted all restrictions on visits and money transfers between American citizens and relatives in Cuba this month, while Raúl Castro said Cuba is prepared to talk with the United States about "everything - human rights, freedom of the press, political prisoners."

Speaking at a conference of inter-American and Caribbean leaders Sunday, Obama said the Cuban leader's declaration was "a sign of progress."

He added that the Cuban government could send a much clearer, more positive signal by releasing political prisoners or reducing fees charged on remittances Americans send to relatives in the country.

But Fidel Castro wrote Wednesday, "There is no doubt that the president misinterpreted Raúl's statements."